1. Field of the Invention
The present invention relates to a high frequency filter employed in the ultra high frequency band (approximately 1 to 100 GHz) covering the frequency range such as microwave and millimeter-wave bands, and particularly to an easily designable, variable-frequency high frequency filter adapted to use a coplanar line resonator and capable of electronically controlling the filter characteristics such as a transmission characteristic, particularly a band characteristic or the like.
2. Description of the Related Arts
A high frequency filter has been widely employed as a functional element imperative for injection and extraction of a desired signal and also suppression and elimination of unwanted signals in transmission and reception apparatuses, in a variety of radio communication facilities, optical fiber high-speed transmission apparatuses and measuring instruments related to the above apparatuses.
Conventionally, the high frequency filters for the microwave and higher-frequency bands have been realized typically through the use of metal waveguides and dielectric resonators. The high frequency filters of a microwave integrated circuit configuration as well have been used in recent years for the purpose of promoting the scaling down of circuitry. The present inventors have proposed in Japanese Patent Laid-open Publication No. 2003-115701 (JP, P2003-115701A) a high frequency filter using a coplanar line resonator, having a microwave integrated circuit configuration and capable of electronically controlling the filter characteristics.
FIG. 1A is a schematic plan view illustrating a conventional high frequency filter using a coplanar line resonator and capable of electronically controlling the filter characteristics. FIG. 1B is a cross-sectional view along line A—A of FIG. 1A.
This high frequency filter is made up through the use of a coplanar line, which is a transmission line of a coplanar configuration, as a resonator. A coplanar configuration refers to a high frequency transmission line made up of a metal conductor formed on one principal surface of a substrate. Hence, a transmission line made of a microstrip line is not included in a transmission line of a coplanar configuration, because a microstrip line needs, in addition of a signal line provided on the one principal surface of the substrate, a ground conductor provided on the other principal surface of the substrate.
Ground conductor 2 is provided on one principal surface of substrate 1 made of dielectric material and a rectangular opening 3 is formed in ground conductor 2. In opening 3, center conductor 4, which functions as a signal line, is provided extending in the longitudinal direction of opening 3. The coplanar line resonator is configured by ground conductor 2 provided on the one principal surface of substrate 1 and center conductor (i.e., signal line) 4 arranged inside opening 3 formed in ground conductor 2, as described above. Here, the resonator is constructed such that the length of center conductor 4 is approximately λ/2, wherein it is assumed that the wavelength corresponding to the intended resonance frequency f0 is λ. Both ends of center conductor 4 are spaced apart from ground conductor 2 at both ends (the left and right ends in the figure) of opening 3 thereby forming electrically open ends. This arrangement allows generation of a standing wave having a null point (i.e., node) of a voltage displacement at the middle point that longitudinally bisects center conductor 4 and maximum (peak) voltage displacements of mutually reverse polarities at both longitudinal ends of center conductor 4 as depicted as curve S illustrated in FIG. 1B, yielding a capability of acting as a resonator. It should be noted that the coplanar line is an unbalanced transmission line that allows traveling of a high frequency electromagnetic wave caused by an electric field, generated between center conductor 4 and ground conductor 2, and a magnetic field induced by the electric field.
Furthermore, in the one principal surface of substrate 1, variable capacitance diodes 5 are arranged individually allocated to both end portions of opening 3, i.e., the gaps between both ends of center conductor 4 and ground conductor 2. In the illustrated example, using solder for example, variable capacitance diodes 5 connect respective ends of center conductor 4 and the edge portions of ground conductor 2 across the end portions of opening 3 with the anodes connected to center conductor 4. To the middle point of the coplanar line resonator, i.e., the middle point that longitudinally bisects center conductor 4, is connected one end of one of supply lines 6a for applying control voltage Vc to variable capacitance diodes 5. The other supply line 6b, i.e., one end of the ground line, is connected to ground conductor 2. The above arrangement allows applying control voltage Vc, which is a reverse voltage (a negative voltage), to the anode of each variable capacitance diode 5 and varying the capacitance values of the diodes.
Input line 7 and output line 8 are provided on the other principal surface of substrate 1 in the respective areas corresponding to both end portions of center conductor 4. Input line 7 is made up of a closed loop section, which surrounds the left end portion, as viewed in the figure, of center conductor 4 and an extension section that extends from the closed loop section to the left end portion, as viewed in the figure, of substrate 1. The closed loop section of input line 7 is provided to traverse center conductor 4 under the neighborhood of the left end, as viewed in the figure, of center conductor 4 and further surround variable capacitance diode 5. Similarly, output line 8 is made up of a closed loop section, which surrounds the right end portion, as viewed in the figure, of center conductor 4 and an extension section that extends from the closed loop section to the right end portion, as viewed in the figure, of substrate 1. The closed loop section of output line 8 is provided to traverse center conductor 4 under the neighborhood of the right end, as viewed in the figure, of center conductor 4 and further surround variable capacitance diode 5. These input line 7 and output line 8 form microstrip line structures together with ground conductor 2 and are electrically connected with the coplanar line, which acts as a resonator, through electromagnetic coupling. The position that input line 7 or output line 8 traverses center conductor 4 is referred to as a transverse point. In this example, the distance between the transverse point of input line 7 and the left end of center conductor 4 is taken to be equal to the distance between the transverse point of output line 8 and the right end of center conductor 4. This distance is denoted as d.
This structure of the resonator allows generating a plurality of resonance points operable as an input/output resonance point in the high frequency filter depending on a boundary condition stipulated on the basis of the positions of input line 7 and output line 8 provided on the other principal surface of the substrate and traversing the coplanar line resonator, for example, the lengths from the transverse points of input line 7 and output line 8 to the ends of center conductor 4. In the above example, because length d between the transverse point and the end of center conductor 4 is the same for input line 7 and for output line 8, basically one input/output resonance point is generated at the frequency corresponding to a wavelength wherein one fourth the wavelength equals d. Specifically, because the length from the transverse point to the tip of the center conductor is one-fourth the wavelength, both ends of the center conductor behave as electrically short-circuit ends as viewed from respective transverse points. Consequently, a high frequency current is created having one-fourth the wavelength equal to that length, yielding an input/output resonance point that causes a voltage fall in the frequency region on the high-frequency side of the resonance characteristic of center conductor 4. Because center conductor 4 functions as a both-end open half-wavelength resonator, and because the distances from respective transverse points to the corresponding ends of center conductor 4 are necessarily shorter than half the length of center conductor 4 itself, the resonance frequency at the input/output resonance point is necessarily higher than the resonance frequency of the coplanar line resonator.
As shown in FIG. 2, in this high frequency filter, attenuation pole P due to the input/output resonance point is created in the frequency region on the high-frequency side of the band characteristic curve (curve T) for the high frequency filter provided with the coplanar line resonator. Consequently, the characteristic curve exhibits a steep gradient of attenuation on the high frequency side of the resonance frequency of the coplanar line resonator f0, as shown by curve U. As a result, the band characteristic of the high frequency filter comes to have a substantially narrowed-down bandwidth, entailing enhancement of an apparent Q value. In the above example, the distances d between both ends of center conductor 4 and the respective transverse points of input line 7 and output line 8 are equal to each other. As a result, the two input/output resonance points are substantially degenerated to one resonance point causing the attenuation level at the attenuation pole P to increase by just that much.
Further, connecting variable capacitance diodes 5 between both ends of the coplanar line resonator, i.e., both ends of center conductor 4, and ground conductor 2 allows variation of resonance frequency f0 to be caused through the variation of the capacitance by means of control voltage Vc. In this arrangement, because variable capacitance diodes 5 are arranged in the electric fields generated between center conductor 4 and ground conductor 2, the capacitance variation of variable capacitance diodes 5 yields equivalently the variation of an electrical length of center conductor 4. In this way, a so-called voltage-controlled high frequency filter is constituted.
In the foregoing high frequency filter, employing a coplanar line resonator of a coplanar structure enables both terminals of variable capacitance diodes 5 to be connected on the same plane to apply the surface mount technology to the mounting of variable capacitance diodes 5. In addition, connecting the supply lines 6a to the middle point that bisects center conductor 4, i.e., connecting the supply lines 6a to the middle point of a half-wavelength resonator, which is a null point (a minimum point) of the voltage displacement, and applying control voltage Vc to the middle point substantially minimizes the influence of the providing of the supply line on the resonance characteristic.
The high frequency filter using the coplanar line resonator of the above structure, however, is configured such that, while both ends of center conductor 4 are spaced apart from ground conductor 2 at both ends of opening 3 electrically to make open ends, variable capacitance diodes 5 are arranged there. Variable capacitance diode 5 varies the value of its capacitance through the application of control voltage Vc from supply lines 6 provided in the middle point of center conductor 4. In this arrangement, for example, if the value of the capacitance of variable capacitance diodes 5 is large, then a high frequency current corresponding to the capacitance of variable capacitance diodes 5 is generated at both ends of center conductor 4 causing the characteristics of the resonator to vary in the direction from an ideal electrical open end to a short-circuit end. As a result, a variation of control voltage Vc causes changes in positions of the maximum voltage displacements at both end portions of center conductor 4 and further causes the null point of the voltage displacement, which should be in the middle point of center conductor 4, to displace making the null point deviate from the middle point. Consequently, the position of one of the supply lines 6a provided at the middle point of center conductor 4 deviates from the null point of the voltage displacement, i.e., the position of the supply line 6a comes to the position where a voltage displacement is present due to the voltage standing wave. As a result, connection of supply line 6a affects the resonance characteristics of the resonator, which makes it difficult to design the resonator. For example, when the capacitance of variable capacitance diodes 5, to which reference control voltage Vco is applied, is taken as a reference capacitance and the central resonance frequency of the resonator for the reference capacitance is denoted as f0, it becomes difficult to grasp the variation of resonance frequency when a control voltage differing by some value from reference control voltage Vco is applied. In addition, the deviation of the position of the null point from the middle point of the resonator brings about difficulty in the foregoing control of the frequency of the input/output resonance point, further entailing difficulty in designing a filter having desired attenuation characteristics.
The arrangement of connecting capacitances created by the variable capacitance diodes with both ends of center conductor 4 acts to lower the maximum voltage value (the magnitude of the voltage displacement) in the point of the maximum voltage displacement of the standing wave induced in center conductor 4. Furthermore, in the case where the capacitance variation characteristics of the pair of variable capacitance diodes 5 against control voltage Vc are different, the balance with respect to the middle point of center conductor 4 is lost, entailing a loss. Due to the above facts, the Q value, which indicates resonance sharpness, lowers and the resonance characteristic of the resonator is degraded.